The present invention relates generally to venting systems for storage tanks. More particularly, the present invention relates to an inwardly opening, mechanically operated vent system used for venting air from storage tanks when fluid is bottom loaded into the storage tank, or replacing air into the storage tank when fluid is unloaded.
Many storage tanks include bottom loading valves which allow the operator to operate the loading valve without going on top of the tank. This presents several advantages, including the gravity outflow of fluid during unloading. Also, the danger of the operator falling off the tank while operating the loading valve is eliminated. However, during loading and unloading a tank it is necessary to provide a vent to allow air to enter the tank during unloading, and allow air to leave the tank while the tank is being loaded.
In fluid bottom loading applications, mechanically operated vents are normally used in connection with the bottom loading valve. The bottom loading valve is installed in the bottom part of the storage tank, while the vent is installed at the top. These vents are commonly installed directly above the bottom loading valve, or in close vicinity thereto. A link, normally a pipe or a bar, is used to connect the bottom loading valve and the vent actuating mechanism so that the vent opens and closes automatically when the bottom operated loading valve is opened and closed. Thus, an equal volume of air will be displaced from or into the tank through the vent as fluid is being loaded or unloaded through the bottom loading valve. This prevents the tank from being burst by pressurization, or collapsed due to depressurization.
The bottom loading valve designs and vents of the prior art are normally closed using shut-off springs. The bottom loading valves are commonly pilot operated valves which are to be opened by the inlet fluid pressure as the pilots are actuated. As the bottom loading valve opens into the tank, the linkage simultaneously pushes the vent valve outward from the tank. In this mode of operation, the fluid pressure force must be sufficient to overcome both the spring forces in the vent and the bottom loading valve.
In the case of a storage tank being toppled or rolled over, the fluid stored in the tank will apply pressure on the vent. Since the previous art vents are outwardly opening, and held closed only by spring force, the pressure from generated by the depth of the fluid or the G-force created as the tank topples can overcome the vent spring force easily. Leakage or spillage may result from this design. This fluid-assisted opening of the vent causing spillage becomes particularly severe when the storage tank is mobile, such as a fluid transport vehicle, which might topple at high speeds and generate significant G-forces.
One solution to this problem is to put a very heavy spring in the vent to prevent it from opening under the fluid pressure as the tank rolls over. However, this necessitates an undesirably high inlet fluid pressure to overcome the combined spring forces to open the bottom loading valve and vent in normal fluid loading and unloading operations. Without adequate venting, the storage tank compartment can rupture or explode due to a significant increase or decrease in the tank internal pressure.
Accordingly, there is a need for a storage tank vent system that prevents fluid from inside the storage tank from spilling through the vent when the tank rolls over or is toppled. Such a vent system should only require a minimum force to allow the bottom loading valve to open at a much lower inlet pressure in normal fluid loading operations. What is also needed is a vent system that can be opened by external atmospheric pressure to compensate for the loss of internal pressure within the storage tank. The present invention fulfills these needs and provides other related advantages.
The present invention resides in an inwardly opening mechanical vent system for use in a fluid tank having a bottom loading valve and a vent formed in an upper tank surface. The vent system comprises a body extending downwardly into the tank from the upper tank surface. A vent valve is associated with the body and configured to seal the vent. Typically, a valve seat encircles the vent and is configured to engage the valve vent to cooperatively seal the vent. A spring is interposed between the vent valve and the body to bias the vent valve upwardly and into a closed position to seal the vent.
A shaft extends downwardly from the vent valve, and a lever mechanism is operably connected thereto. The body typically substantially surrounds the shaft, while permitting the shaft to reciprocally move through the body. The body includes apertures which allow fluid communication, of the air or vapor contents of the tank with the opened vent so that these can escape from the tank during loading.
The lever mechanism includes a scissor-hinge assembly connected to a trunnion of the body. The scissor-hinge assembly comprises a counter lever pivotally attached to the trunnion intermediate first and second ends thereof and an upper and lower hitch. The upper hitch is connected at one end to the shaft, and pivotally connected at the other end to the first end of the counter lever. The lower hitch is connected at one end to a slide associated with a guide extending from the body. The lower hitch is pivotally attached at the other end to a second end of the counter lever.
A linkage rod extends upwardly from the bottom loading valve, and is capable of actuating the lever mechanism to lower the shaft and vent valve into the tank to open the vent when the bottom loading valve is opened. More particularly, the guide is associated with the linkage rod in such a manner so as to permit the linkage to reciprocally move upwards into contact with the slide, and downwards away from the slide, while preventing substantial lateral movement of the linkage rod. Typically, the guide comprises a rod extending downwardly from the body, and the linkage rod includes a tubular portion that substantially surrounds the rod. Thus, as the bottom loading valve is opened, the linkage rod is moved upwardly into contact with the slide, causing the scissor-hinge assembly to collapse, and pull the vent valve and shaft downwardly to open the vent.
The inwardly opening mechanical vent system uses a shut-off spring with minimum force to allow the bottom loading valve to open at a much lower inlet pressure in normal fluid loading operations. When a vacuum is created in the storage tank, such as when unloading fluid from the bottom loading valve or leakage from the tank, atmospheric pressure can easily open the vent into the tank to compensate for the loss of internal pressure. Also, the invention prevents fluid from inside the storage tank from spilling through the vent when the tank rolls or topples over as the fluid pressure would cause the vent valve to move outward and further seal the vent.